Semiconductive jackets are useful, for example, in medium and high voltage underground and submarine power cable applications.
The National Electrical Safety code requires that underground power cables with electrical insulation jackets must be grounded, at least, at approximately every 400 m. The grounding secures the system safety and also reduces the energy losses in a cable system during operation.
In order to avoid the need of periodic grounding and increase the system safety, the outer protective jacket of a cable may be coated with a layer of graphite in a powder form in order to make the surface of the outer jacket semiconductive, see e.g. US2010231228. Thus, by making the surface of the outer jacket semiconductive, the cable is grounded throughout the total length. When the cable is grounded throughout the total length, the cable system is advantageously diagnosable and any cable defects may become detectable by applying a high voltage of constant polarity (DC) on the semiconductive layer, or by performing a partial discharge test during production and/or after installation and/or even during operation. Furthermore, this may provide protection of the cable from lightening. However, it is in practice difficult to form a uniform graphite layer on the surface of the jacket and the formed graphite layer is also often mechanically vulnerable since the adhesion of graphite to the jacket tends to be weak. Alternatively, conductive varnish, which adheres stronger than graphite but has poor mechanical properties, may be applied on the surface of the jacket.
To improve the reliability of the semiconductive layer, semiconductive compositions, which may be extruded directly as an outer jacket layer, have been proposed.
The extrudable semiconductive compositions may be thermoplastic and normally comprise polyolefin, such as low density polyethylene (LDPE), linear low density polyethylene (LLDPE), medium density polyethylene (MDPE), high density polyethylene (HDPE) or a mixture comprising ethylene alkyl acrylate copolymer or ethylene vinyl acetate copolymer as a base resin, a conductive filler and additives, see e.g. WO2011149463, US2010231228 or U.S. Pat. No. 6,514,608.
However, to meet the physical requirements of a conductive jacket, it is required that the semiconductive compositions exhibit further improved mechanical properties such as a high shore D value, mechanical flexibility and high environmental stress cracking resistance (ESCR).
Traditionally, the conductive filler, such as carbon black, which is comprised in the semiconductive compositions, has been kept at a low content level to minimize its interference to the inherent mechanical performance of the base resin. For example, special high conductive carbon blacks, such as Ketjenblack, or other highly conductive fillers, have preferably been used in traditional semiconductive compositions in order to meet the electric requirements with regard to a low volume resistivity (VR) or a high conductivity, of a semiconductive jacket with a low content of a conductive filler.
Alternatively, a phase separation base resin system has also been used to maintain the conductive fillers in a continuous minor phase and to form a semiconductive composition. In the semiconductive composition formed with a phase separation base resin system, the VR value may be kept at a low level also with a low content of the conductive fillers (see U.S. Pat. No. 6,514,608).
However, the traditional approaches still have significant drawback such as the high cost of the special conductive fillers, or high extrusion process dependence of conductivity due to the nature of phase separation dynamics.